Mybatis插件原理分析（一）

我们首先介绍一下Mybatis插件相关的几个类，并对源码进行了简单的分析。

Mybatis插件相关的接口或类有：Intercept、InterceptChain、Plugin和Invocation，这几个接口或类实现了整个Mybatis插件流程。

Interceptor：一个接口，是实现自己功能需要实现的接口

源码如下：

/** * @author Clinton Begin */public interface Interceptor {  //在此方法中实现自己需要的功能，最后执行invocation.proceed()方法，实际就是调用method.invoke(target, args)方法，调用代理类  Object intercept(Invocation invocation) throws Throwable;  //这个方法是将target生成代理类  Object plugin(Object target);  //在xml中注册Intercept是配置一些属性  void setProperties(Properties properties);}

InterceptorChain：有一个List<Interceptor> interceptors变量，来保存所有Interceptor的实现类

源码如下：

/** * @author Clinton Begin */public class InterceptorChain {  //插件拦截器链  private final List<Interceptor> interceptors = new ArrayList<Interceptor>();  //把target变成代理类，这样在运行target方法之前需要运行Plugin的invoke方法  public Object pluginAll(Object target) {    for (Interceptor interceptor : interceptors) {      target = interceptor.plugin(target);    }    return target;  }  public void addInterceptor(Interceptor interceptor) {    interceptors.add(interceptor);  }    public List<Interceptor> getInterceptors() {    return Collections.unmodifiableList(interceptors);  }}

Invocation：一个比较简单的类，主要的功能就是根据构造函数类执行代理类

源码如下：

/** * @author Clinton Begin */public class Invocation {  private Object target;  private Method method;  private Object[] args;  public Invocation(Object target, Method method, Object[] args) {    this.target = target;    this.method = method;    this.args = args;  }  public Object getTarget() {    return target;  }  public Method getMethod() {    return method;  }  public Object[] getArgs() {    return args;  }  //其实mybatis的Interceptor最终还是调用的method.invoke方法  public Object proceed() throws InvocationTargetException, IllegalAccessException {    return method.invoke(target, args);  }}

Plugin：Mybatis插件的核心类，它实现了代理接口InvocationHandler，是一个代理类

源码详解如下：

/** * @author Clinton Begin *///这个类是Mybatis拦截器的核心,大家可以看到该类继承了InvocationHandler//又是JDK动态代理机制public class Plugin implements InvocationHandler {  //目标对象  private Object target;  //拦截器  private Interceptor interceptor;  //记录需要被拦截的类与方法  private Map<Class<?>, Set<Method>> signatureMap;  private Plugin(Object target, Interceptor interceptor, Map<Class<?>, Set<Method>> signatureMap) {    this.target = target;    this.interceptor = interceptor;    this.signatureMap = signatureMap;  }  //一个静态方法,对一个目标对象进行包装，生成代理类。  public static Object wrap(Object target, Interceptor interceptor) {//首先根据interceptor上面定义的注解 获取需要拦截的信息    Map<Class<?>, Set<Method>> signatureMap = getSignatureMap(interceptor);    Class<?> type = target.getClass();    Class<?>[] interfaces = getAllInterfaces(type, signatureMap);//如果长度为>0 则返回代理类 否则不做处理    if (interfaces.length > 0) {  //创建JDK动态代理对象      return Proxy.newProxyInstance(          type.getClassLoader(),          interfaces,          new Plugin(target, interceptor, signatureMap));    }    return target;  }  //在执行Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类的方法时会调用这个方法  @Override  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {    try {  //通过method参数定义的类 去signatureMap当中查询需要拦截的方法集合      Set<Method> methods = signatureMap.get(method.getDeclaringClass());  //判断是否是需要拦截的方法，如果需要拦截的话就执行实现的Interceptor的intercept方法，执行完之后还是会执行method.invoke方法，不过是放到interceptor实现类中去实现了      if (methods != null && methods.contains(method)) {        return interceptor.intercept(new Invocation(target, method, args));      }  //不拦截 直接通过目标对象调用方法      return method.invoke(target, args);    } catch (Exception e) {      throw ExceptionUtil.unwrapThrowable(e);    }  }  //根据拦截器接口（Interceptor）实现类上面的注解获取相关信息  private static Map<Class<?>, Set<Method>> getSignatureMap(Interceptor interceptor) {//获取注解信息    Intercepts interceptsAnnotation = interceptor.getClass().getAnnotation(Intercepts.class);    // issue #251//为空则抛出异常    if (interceptsAnnotation == null) {      throw new PluginException("No @Intercepts annotation was found in interceptor " + interceptor.getClass().getName());          }//获得Signature注解信息    Signature[] sigs = interceptsAnnotation.value();    Map<Class<?>, Set<Method>> signatureMap = new HashMap<Class<?>, Set<Method>>();//循环注解信息    for (Signature sig : sigs) {  //根据Signature注解定义的type信息去signatureMap当中查询需要拦截方法的集合      Set<Method> methods = signatureMap.get(sig.type());  //第一次肯定为null 就创建一个并放入signatureMap      if (methods == null) {        methods = new HashSet<Method>();        signatureMap.put(sig.type(), methods);      }      try {//找到sig.type当中定义的方法 并加入到集合        Method method = sig.type().getMethod(sig.method(), sig.args());        methods.add(method);      } catch (NoSuchMethodException e) {        throw new PluginException("Could not find method on " + sig.type() + " named " + sig.method() + ". Cause: " + e, e);      }    }    return signatureMap;  }  //根据对象类型与signatureMap获取接口信息  private static Class<?>[] getAllInterfaces(Class<?> type, Map<Class<?>, Set<Method>> signatureMap) {    Set<Class<?>> interfaces = new HashSet<Class<?>>();//循环type类型的接口信息 如果该类型存在与signatureMap当中则加入到set当中去    while (type != null) {      for (Class<?> c : type.getInterfaces()) {        if (signatureMap.containsKey(c)) {          interfaces.add(c);        }      }      type = type.getSuperclass();    }//转换为数组返回    return interfaces.toArray(new Class<?>[interfaces.size()]);  }}

对Mybatis插件有一定了解的人应该知道，插件拦截的类是Executor、ParameterHandler、ResultSetHandler和StatementHandler的实现类，为什么会这样呢？在Configuration类中看一下代码就明白了，可以看到在初始化Executor、ParameterHandler、ResultSetHandler和StatementHandler都会调用interceptorChain.pluginAll（）这个函数，其实这样之后各个接口的实现类就被代理类生成为目标类了。

 public ParameterHandler newParameterHandler(MappedStatement mappedStatement, Object parameterObject, BoundSql boundSql) {    ParameterHandler parameterHandler = mappedStatement.getLang().createParameterHandler(mappedStatement, parameterObject, boundSql);    parameterHandler = (ParameterHandler) interceptorChain.pluginAll(parameterHandler);    return parameterHandler;  }  public ResultSetHandler newResultSetHandler(Executor executor, MappedStatement mappedStatement, RowBounds rowBounds, ParameterHandler parameterHandler,      ResultHandler resultHandler, BoundSql boundSql) {    ResultSetHandler resultSetHandler = new DefaultResultSetHandler(executor, mappedStatement, parameterHandler, resultHandler, boundSql, rowBounds);    resultSetHandler = (ResultSetHandler) interceptorChain.pluginAll(resultSetHandler);    return resultSetHandler;  }  public StatementHandler newStatementHandler(Executor executor, MappedStatement mappedStatement, Object parameterObject, RowBounds rowBounds, ResultHandler resultHandler, BoundSql boundSql) {    StatementHandler statementHandler = new RoutingStatementHandler(executor, mappedStatement, parameterObject, rowBounds, resultHandler, boundSql);    statementHandler = (StatementHandler) interceptorChain.pluginAll(statementHandler);    return statementHandler;  }  public Executor newExecutor(Transaction transaction) {    return newExecutor(transaction, defaultExecutorType);  }  public Executor newExecutor(Transaction transaction, ExecutorType executorType) {    executorType = executorType == null ? defaultExecutorType : executorType;    executorType = executorType == null ? ExecutorType.SIMPLE : executorType;    Executor executor;    if (ExecutorType.BATCH == executorType) {      executor = new BatchExecutor(this, transaction);    } else if (ExecutorType.REUSE == executorType) {      executor = new ReuseExecutor(this, transaction);    } else {      executor = new SimpleExecutor(this, transaction);    }    if (cacheEnabled) {      executor = new CachingExecutor(executor);    }    executor = (Executor) interceptorChain.pluginAll(executor);    return executor;  }